A base station in a wireless communication network provides wireless communications services to a large plurality of User Equipment (UE) within its service area, or cell. Each base station generally includes a scheduling function, which schedules downlink transmissions to numerous UE according to several criteria (e.g., amount of data for each UE, channel quality reported by each UE, UE bit rate capability, amount of data recently transmitted to a UE, and the like). Efficient scheduling improves overall throughput. For example, in proportionally fair scheduling, more data is transmitted to UE experiencing the best channel conditions, with UE experiencing poor channel conditions being served just often enough to maintain an acceptable level of performance. A recent development in wireless communication network technology is the cooperative scheduling of traffic between multiple cells, which can reduce inter-cell interference.
Radio Frequency (RF) signals transmitted across an air interface experience interference and noise, distorting the signal and introducing errors in the data extracted from the signal at a receiver. Accordingly, error detection and correction techniques are critical to digital wireless communications. Indeed, to maximize overall performance, system parameters are often adjusted to achieve a predetermined, non-zero error rate, e.g., 10% Block Error Rate (BLER).
A simple error detection scheme is to add a parity bit to a block of bits, the parity bit value set to ensure that the number of bits with the value 1 in the block is even or odd. The receiver performs a parity check by counting the number of 1's in the block, and comparing the result to the parity bit. A more advanced system for error detection is the Cyclic Redundancy Check (CRC). A type of hash function is performed on a block of data, and the result is appended to and transmitted with the block. A receiver performs the same calculation, and compares its result to the received CRC value to detect errors. Numerous other error detection schemes are known in the art; for example, Forward Error Correction (FEC) coding encodes sufficient redundancy into a data block for the receiver to perform not only error detection, but also unassisted correction of some errors.
Automatic Repeat reQuest (ARQ) is simple error correction scheme in which a receiver, upon detecting an error, requests a retransmission of a block of data. The receiver may explicitly acknowledge each data block, by transmitting an acknowledgment (ACK) if no errors are detected or a negative acknowledgment (NAK) if the block was received with one or more errors. Alternatively, ACKs may be implicit, with the receiver only transmitting a NAK in the case of a detected error. The transmitter retains a copy of each transmitted data block until the receipt of an ACK/NAK, or in the case of implicit acknowledgment, until either a NAK is received or a timeout occurs.
Upon receiving a NAK, the transmitter re-transmits coded data based on the original data block. When the retransmission is based on repetition of previously sent coded bits, the transmitter is operating in a Chase combining HARQ protocol. When the retransmission contains coded bits unused in previous transmission attempts, it is operating in an incremental redundancy HARQ protocol. The receiver may replace the erroneously received data block with the re-transmitted one, or may combine the two, and additionally utilize FEC codes, to correct the errors. Since the re-transmission itself may be received with errors, it may be NAK'ed, prompting another re-transmission. Various wireless communication system protocols define a maximum number of re-transmission attempts, after which higher-layer error recovery techniques are invoked. In the following, we will use re-transmission to mean both the case of repetition of previously sent coded bits, and the case of coded bits unused in previous transmission attempts, and the combination of the two.
An ARQ or HARQ scheme is simple to implement at both the transmitter and receiver, minimally degrades performance, adds little overhead to the transmitted signals, and not only allows the system to recover from inevitable errors, but allows the system to be purposefully operated with a desired non-zero error rate. However, while the overall error rate may be tuned (e.g., 10% BLER), the timing of individual block errors, and hence the timing of NAKs and HARQ re-transmissions, is random. This random injection of required transmissions prevents the scheduler from operating as efficiently as it could if all its transmissions in a given frame were deterministic. HARQ re-transmissions also impede the ability to coordinate schedulers across cells.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.